The present invention relates to the gas assisted injection molding of plastic parts. In particular the present invention involves an improvement in the control of air during the injection process.
Gas assisted injection molding is well known in the art. Generally, hot plastic resin is injected into a mold and gas is injected into the mold to xe2x80x9cpushxe2x80x9d the molten resin to completely and uniformly cover the surface of the mold. The gas is then vented, the mold opened and the part removed.
The success of the xe2x80x9cgas assistxe2x80x9d process can depend greatly upon the injection pressure of the gas. Baxi, U.S. Pat. No. 4,935,191 describes injecting gas at a pressure at least equal to the pressure of the plastic, without controlling the flow of gas. Conversely Hendry U.S. Pat. Nos. 4,855,094 and 4,943,407 provide a gas charge lower than the plastic injection pressure. Others, such as in Loren U.S. Pat. Nos. 5,118,455 and 5,039,463 have endeavored to plot detailed gas pressure profiles to describe preferred processes for controlling the gas injection pressure always providing a source pressure higher than the initial injection pressure. A further example is U.S. Pat. No. 5,141,682 to Steinbichler which provides dual gas supply and dual hydraulic reservoirs. The focus of these efforts have been to provide a surplus volume of gas so that the desired pressure profile can be run and the reservoir pressure will never drop below the pressure desired. Each of these efforts fail to take full advantage of the full storage pressure, as they regulate the pressure down prior to injection. While it is known to increase pressure during the xe2x80x9choldxe2x80x9d cycle in which the plastic cures against the mold surface, no one appears to have recognized the advantage of a high pressure burst to begin the gas injection. The convoluted efforts to control gas pressure (including controlling volume) were partially inspired by failures when gas was injected at too high a pressure, which would pierce through the molten resin much like a child over blowing a chewing gum bubble, and creating pressure within the mold which would resist the injection of molten plastic.
These and other advantages of the present invention will be readily apparent from the drawings, discussions and description, which follow.
While preferred gas injection profiles and injection control can vary from application to application, it has been determined that a brief burst of gas injected at the beginning of the gas injection cycle facilitates the gas injection process and generally improves the molding process. While molten resin is fluid, it is viscous to the point of requiring the gas pressure to flow to completely cover the mold surface. In the past failures, the high pressure injection was not regulated in time, and thus the gas pressure would shear the molten material. Where others regulated the initial injection pressure downward, the present invention regulates the time of the initial injection.
The initial burst of gas, at whatever pressure is available, rapidly displaces the charge of molten resin injected prior to the gas injection, providing an impulse which overcomes the friction the molten plastic incurs in the inlet. Although the effects upon the physical properties of the plastic resulting from the burst cannot be fully explained, when the pressure is reduced to a typical injection pressure, the plastic more easily flows and completely covers the mold than if the initial burst was not provided. Further, the initial gas burst allows the plastic to be injected at a more rapid rate, thus allowing the plastic to travel farther while hot, when it is in a more fluid state. Thus, the remaining fill out portion of the gas injection cycle may be conducted at a relatively lower pressure.
Critical to the process is the control of the initial burst in time, so that the material does not shear and the xe2x80x9cblowoutxe2x80x9d condition occurs. Once the initial burst is complete, the remainder of the gas injection cycle (fill out, hold, vent) is conducted to optimize the molding process for the given application (part size and thickness, material used).
Accordingly, the object of the present invention is to utilize the stored pressure in a gas assisted injection molding operation to provide an initial burst of gas at the stored pressure at the beginning of the gas injection stage.